Voltage selectors typically comprise two or more voltage supply points that may be selected using a selector switch. The selector switch may be implemented using diodes, MOSFETs or comparator circuits.
Referring to FIG. 1, a diode based voltage selector 100 is shown. The diode based voltage selector 100 comprises a first diode 105 and a second diode 110. The anode of the first diode 105 is connected to a first supply voltage V1 and the anode of the second diode 110 is connected to a second voltage supply V2. Further, output voltage Vmax (V1, V2) is taken from a common node connecting the cathodes of the first diode 105 and the second diode 110. More specifically, the output of the diode based voltage selector 100 is based on the maximum voltage between V1 and V2. However, the output voltage Vmax (V1, V2) is lower than the maximum input voltage by Vd, where Vd is the forward drop voltage (approximately equal to 0.7V) of a diode. For example, if V1 is greater than V2, then Vmax (V1, V2)=V1−Vd. In another example, if V2 is greater than V1, then Vmax (V1, V2)=V2−Vd. The diode based voltage selector 100 has the advantage that only a small reverse leakage current is drawn from the maximum supply voltage.
Referring to FIG. 2, a back to back PMOS voltage selector 200 is shown. The PMOS voltage selector 200 comprises a first P-MOSFET 205 and a second P-MOSFET 210, where the first P-MOSFET 205 is connected to a first supply voltage V1 and the second P-MOSFET 210 is connected to a second supply voltage V2. More specifically, the first supply voltage V1 is provided at the drain terminal of the first P-MOSFET 205 and the second supply voltage V2 is provided at the drain terminal of the second P-MOSFET 210. Further, the bulk terminal and the source terminal of the first P-MOSFET 205 are shorted as shown. Similarly, the bulk terminal and source terminal of the second P-MOSFET 210 are also shorted. The drain terminal of the first P-MOSFET 205 is further connected to the gate terminal of the second P-MOSFET 210. Similarly, the drain terminal of the second P-MOSFET 210 is further connected to the gate terminal of the first P-MOSFET 205. Furthermore, the source terminals of the first P-MOSFET 205 and the second P-MOSFET 210 are connected together to form an output node.
If the first supply voltage V1 is greater than the second supply voltage V2 by a threshold voltage VTH, then the first P-MOSFET 205 is switched ON and the second P-MOSFET 210 is switched OFF, thereby providing V1 at the output node. Otherwise, if the second supply voltage V2 is greater than the first supply voltage V1, then the second P-MOSFET 210 is switched ON and the first P-MOSFET 205 is switched OFF, thereby providing V2 at the output node. However, if the input voltages, the first supply voltage V1 and the second supply voltage V2 are very close to each other, then both the first P-MOSFET 205 and the second P-MOSFET 210 have very small gate-source voltage. As a result, both the first P-MOSFET 205 and the second P-MOSFET 210 fail to provide high load currents when required. Further, if a high load current is drawn from the voltage selector, then the bulk diodes of the first P-MOSFET 205 and/or the second P-MOSFET 210 conduct giving V1−VTH or V2−VTH at the output node.
Referring to FIG. 3, a comparator-based voltage selector 300 is shown. In the comparator-based voltage selector 300, a comparator 305 compares the first voltage V1 and the second voltage V2. Further, based on the output of the comparator, a first P-MOSFET 310 or a second P-MOSFET 315 is switched ON. Subsequently, the higher voltage between V1 and V2 appears at an output node. It should be understood that the voltage dropout between the input and the output is equal to the drain-source voltage VDS of the P-MOSFET in ON state. Further, the comparator 305 constantly draws a quiescent current from one of the first voltage V1 and the second voltage V2. For example, if the first voltage V1 is supplied by a battery (not shown) and the second voltage V2 is supplied by a wall voltage adaptor working of the mains supply (not shown, from here on referred to as adaptor), then drawing of quiescent current from the battery would results in faster draining of the battery. In order to overcome this shortcoming, the quiescent current of the comparator 305 has to be reduced.